1. Field of the Invention
The present invention generally relates to an electromagnetic cooking apparatus capable of heating food in a metal pan by utilizing eddy currents occurred in the metal pan. More specifically, the present invention is directed to an electromagnetic cooking apparatus capable of uniformly heating the food even under low power consumption, and also capable of quickly detecting various sorts of heating loads.
2. Description of the Related Art
Various types of electromagnetic cooking apparatuses for utilizing an electromagnetic induction effect to heat food or the like have been developed and marketed with the following advantages. No flame is needed to heat food or the like, i.e., a safety factor in view of fire problems. A top plate to mount an article to be electromagnetically heated, such as a metal pan, can be made of a crystallized glass, for clean cooking. Furthermore higher heat efficiency can be achieved.
In FIG. 1, there is shown a circuit diagram of one conventional electromagnetic cooking apparatus. A predetermined DC voltage derived from a DC (direct current) power supply circuit 101 is applied to a DC-to-AC inverter circuit 103. While a transistor 113 is turned ON/OFF by a drive circuit 115, both a heating coil 107 and a resonance coil 109 are set to a series resonance condition, and a heating operation is carried out in such a manner that eddy currents are produced in an article to be heated such as an iron pan 100 by the electromagnetic induction effect caused by the magnetic flux produced in the heating coil 107.
A pulse width modulation circuit 119 including an oscillator (not shown in detail) adjusts an oscillation period of an oscillating pulse derived from the oscillator in response to a timing pulse from a voltage feedback circuit 117, and also modulates a pulse width of the oscillating pulse in response to a signal from signals derived from an input setting circuit 121 and an ON-time setting circuit 123. The drive circuit 115 will turn ON the switching transistor 113 for a time duration corresponding to the pulse width of the PWM (pulse width modulated) pulse signal from the PWM circuit 119.
An input current monitoring circuit 127 outputs to a load detecting circuit 125, a signal corresponding to an input current from an AC power supply unit, namely a current "ic" flowing through an inverter circuit 103 based upon a detection signal from a current transformer CT electromagnetically coupled to the AC power supply unit.
The load detecting circuit 125 monitors the loading condition in response to the signal corresponding to the current "ic" from the input current monitoring circuit 127. As shown in FIG. 2A, for instance, since the proper current "ic" flows through the heating coil 107 on which the iron pan 100 has been mounted, it is judged that the proper load is loaded on the heating coil 107 and thus the operation of the pulse width modulation circuit 119 is continued. As represented in FIGS. 2A and 2C, when either an aluminum pan (not shown in detail), or no pan is mounted on the heating coil 107, the current "ic" flowing therethrough becomes small, or a regenerative current "id" having no heating function flows through the heating coil, 107. It is therefore judged that a no loading condition, or an improper load, is loaded applied to the heating coil 10. Thus, operation of the pulse width modulation circuit 119 is interrupted, to prohibit the heating operation by the heating coil 107.
In another conventional electromagnetic cooking apparatus shown in FIG. 3, an initialization circuit 131 is actuated when a power supply unit is energized, and an oscillation stopping circuit 135 is operated for a predetermined time duration set by an oscillation stopping timer 133 so as to stop the oscillation by the DC/AC inverter 103. Thereafter, when the oscillation stopping circuit 135 has recovered, the voltage "V.sub.TON " which has been set by an ON-time setting circuit 123 is applied to the pulse width modulation circuit 119. When the pulse width modulation circuit 119 outputs a pulse signal having a pulse width corresponding to the voltage V.sub.TON, the switching transistor 113 is turned ON for a time duration corresponding to the pulse width of this pulse signal by the drive circuit 115. As a result, based upon the value of the voltage Y.sub.TON, the ON-time of transistor 113 is set. Thus, the switching transistor 113 is turned ON/OFF based upon the above-described pulse signal so that the RF (radio frequency) current flows through the heating coil 107 in order to heat the metal pan 100.
The load detecting circuit 125 monitors whether or not the proper load is loaded on the heating coil 107. As illustrated in FIG. 4A, in case that the voltage "V.sub.I " corresponding to the input current supplied from the AC power supply unit exceeds over the voltage "V.sub.TON ", a judgment is made that the proper load is loaded on the heating coil 107, whereby the heating operation is continued.
Conversely, when the voltage "V.sub.I " is lower than the voltage "V.sub.TON ", another judgment is made that an improper load, e.g., no load or an aluminum pan, is loaded on the heating coil 107. In this case, as represented in FIG. 4B, there is a problem that it will take a time duration of, e.g., 300 milliseconds until such an improper loading condition is detected while the voltage V.sub.TON gradually increases and then the voltage V.sub.I becomes lower than the voltage V.sub.TON.
In addition, as illustrated in FIG. 5A, when the input power to the DC/AC inverter 103 is high, the collector-to-emitter voltage of the switching transistor 113 employed in this inverter 103, namely the resonance voltage "V.sub.CE " becomes a sinusoidal waveform during the turn-OFF period of this switching transistor 113, wherein the collector current "ic" of the switching transistor 113 is increased in a linear form within the ON-time period "T.sub.ON " of the switching transistor 113. To the contrary, when the input power to the DC/AC inverter 103 is set low, as shown in FIG. 5B, the resonance voltage "V.sub.CE " does not lower to zero volts and, thus, a predetermined potential is produced just before the switching transistor 113 is turned ON. This potential causes the transistor 113 to short circuit so that a short circuit current "Is" flows through the switching transistor 113. As a consequence, power loss in the switching transistor 113 becomes high.
As represented in FIG. 6A, according to a conventional electromagnetic cooling apparatus having such specifications that the input voltage is set to 100 V, and the input power is selected to be 1.2 KW at its maximum, when the input power is set to a low value, a the power loss "W.sub.LOS " in the switching transistor 113 increases. Then, as the minimum input power, the input power can be reduced to approximately 300 watts. If this input power of 300 W is further lowered, the oscillating (switching) time period of the DC/AC inverter circuit 103 may be controlled in a second time period. For instance, the switching operation of the DC/AC inverter circuit 103 must be turned ON for 1 second, and turned OFF for 1 second.
There is a limitation in the maximum resonance voltage "V.sub.CE " of the switching transistor 113 in the DC/AC inverter circuit of the conventional electromagnetic cooking apparatus having the input voltage of 200 V and the maximum input power of 2 KW, due to the rated voltage of this switching transistor. When, for instance, a bipolar type MOSFET, such as an IGBT (Insulated-Gage Bipolar Transistor), is employed as the switching transistor, and switched at a frequency of 25 KHz, the collector voltage thereof is limited to 1,000 volts or below under the normal operating condition since the maximum rated collector voltage of the switching transistor is about 1,400 volts. Furthermore, in the case of an input voltage of 200 V, the DC power source voltage applied from the DC power supply circuit is two times higher than that of the 100 V input voltage specification. Since the resonance voltage V.sub.CE is a voltage corresponding to a half time period of an attenuated waveform which is converged to the DC power source voltage, the resonance voltage "V.sub.CE " of the 200 V input voltage specification is not so lowered as compared with that of the 100 V input voltage. Under the above-described circumstances, when the input power to the DC/AC inverter circuit is set to a lower value in case of the electromagnetic cooking apparatus having the 200 V input voltage specification, the practical minimum input power may not be selected to be lower than 1,000 watts, as illustrated in the graphic representation of FIG. 6B, because the switching transistor 113 may be destroyed due to an occurrence of such a short circuit current.
If, however, this input power of the 200 V input specification is further reduced to about 150 W, the oscillating time period of the DC/AC inverter circuit is controlled in such a manner that the operation of the inverter circuit is turned ON for, e.g., 17 seconds. In other words, the DC/AC inverter circuit 103 is operated only for 3 seconds, and the DC/AC inverting operation thereof must be interrupted for a longer time period, say 17 seconds, in order to achieve the above-described lower input voltage operation.
Such a blocking operation of the DC/AC inverter circuit has the following problems.
In the conventional electromagnetic cooking apparatus having the power source voltage of 100 V and the maximum input power of 1.2 KW, the operation of the DC/AC inverter circuit is turned ON/OFF at the ratio of 1:1 under the condition that the input power is controlled to set 300 W in the PWM (pulse width modulation) control mode. In other words, the inverting operation of the AD/AC inverter circuit is turned ON for 1 second and turned OFF for 1 second. Similarly, in case of the conventional electromagnetic cooking apparatus having the power source voltage of 200 V and the maximum input power of 2 KW, to realize the input power of 800 W, while the input power of the DC/AC inverter circuit is controlled in the PWM control mode to set 800 W, the inverting operation, namely the oscillation time period of the DC/AC inverter circuit is switched at the ratio of 8 to 2. That is, the inverting operation of the inverter circuit is turned ON for 4 seconds and subsequently turned OFF for 1 second. In accordance with the similar control method, to realize the inverter circuit is turned ON/OFF at the ratio of 3 to 17, namely turned ON for 3 seconds and thereafter turned OFF for 17 seconds. Such an ON/OFF control can be applied to either 100 V or 200 V of the power supply voltage in principle, as previously described.
However, to achieve such a lower input power of e.g., 150 watts in the conventional electromagnetic cooking apparatus, the oscillating period namely the switching operation of the DC/AC inverter circuit, must be turned ON/OFF for considerable lengths. As a result, the heating intervals between the succeeding heating operations become so long that the temperature of the article, such as food to be headed can hardly be maintained constant. Accordingly, there are temporal fluctuations in the temperature of the food, resulting in deterioration of the cooking capabilities by the electromagnetic cooking apparatus.
Under these circumstances an electromagnetic cooking apparatus capable of preventing this by quickly judging whether or not the proper load is loaded on the heating coil is needed. In the specific case that the input power to the DC/AC inverter circuit is set to a low value under the higher power supply voltage, there is another drawback that the switching element of the inverter circuit may break down unless the loading condition of the heating coil is quickly adjusted.